CH4 Gas Extraction by CO2: Substitution in Clathrate Hydrate through Bimolecular Iteration**Supported by the National Key R&D Program of China under Grant Nos 2016YFA0401503 and 2018YFA0305700, the National Natural Science Foundation of China under Grant Nos 11575288, 11775011, 21703006, 21773005, and the Youth Innovation Promotion Association of Chinese Academy of Sciences under Grant No 2016006. J. Zhu was supported by

Abstract

Methane clathrate hydrate (MCH) is a promising energy resource, but controllable extraction of CH4 from MCH remains a challenge. Gradually replacing CH4 in MCH with CO2 is an attractive scheme, as it is cost efficient and mitigates the environmentally harmful effects of CO2 by sequestration. However, the practicable implementation of this method has not yet been achieved. In this study, using in situ neutron diffraction, we confirm that CH4 in the 51262 cages of bulk structure-I (sI) MCH can be substituted by gaseous CO2 under high pressure and low temperature with a high substitution ratio (∼44%) while conserving the structure of the hydrate framework. First-principles calculations indicate that CO2 binds more strongly to the 51262 cages than methane does, and that the diffusion barrier for CH4 is significantly lowered by an intermediate state in which one hydrate cage is doubly occupied by CH4 and CO2. Therefore, exchange of CO2 for CH4 in MCH is not only energetically favorable but also kinetically feasible. Experimental and theoretical studies of CH4/CO2 substitution elucidate a method to harness energy from these combustible ice resources.